Several vehicle control systems, which are used to augment the driving capability of a vehicle operator, currently exist. Those control systems include stability control systems. Example stability control systems are electronic stability control (ESC) systems or sometimes referred to as yaw stability control (YSC) systems. Systems of this kind are also sometimes called ESP (Electronic Stability Program) systems or DSTC (Dynamic Stability Traction Control) systems.
The stability control systems are utilized to maintain controlled and stable vehicle operations for improved vehicle and occupant safety. The stability control systems are often used to maintain control of a vehicle following a desired travel direction, to prevent the vehicle from spinning out and help the driver maintain directional stability when cornering.
This function is usually enabled by braking one or more of the wheels if a lateral slide or skidding is detected, but may also be achieved through reducing engine torque, or varying the driving torque at individual wheels or axles so as to generate an active tire force difference.
Interventions as above are usually performed as a function of yaw rate error, where the yaw rate error is determined as the difference between a yaw rate target and a sensed yaw rate. The yaw rate target is normally calculated from a steering wheel angle, which may be considered indicative of the driver intent, and the vehicle velocity using a single track vehicle model, also called bicycle model.
Existing stability control systems are designed to correct undesired vehicle motion caused by variations in tire cornering stiffness. Cornering stiffness is a tire property that describes the cornering behavior of a vehicle tire by relating its side slip angle to a produced lateral friction force.
A challenge related to cornering stiffness is that it may vary significantly for different tires, e.g. a summer tire vs. a winter tire or a low profile tire vs. a high profile tire, but it also varies as the tire ages. For active safety stability control systems, such as e.g. an AYC, ESC, ESP, or YSC system, these variations may cause problems as the vehicle's true handling characteristics change over time. Such variations are likely to cause unnecessary system interventions.
Hence, it is of interest to estimate cornering stiffness online in order to counteract such issues.
It is previously known to account for variations in tire cornering stiffness, as e.g. exemplified by document U.S. Pat. No. 7,774,103, which discloses a system for estimating vehicle side-slip in the linear vehicle operating region is disclosed that includes updating front and rear cornering stiffness parameters. The system includes a first state observer processor that employs a bicycle model for generating yaw acceleration and lateral acceleration signals. The first state observer processor receives sensor signals from a vehicle speed sensor and a hand-wheel angle sensor. The system calculates yaw acceleration and lateral acceleration and compares them to measured yaw rate and lateral acceleration signals to generate yaw acceleration and lateral acceleration error signals. The error signals are sent to a parameter estimation processor that calculates an updated front cornering stiffness and rear cornering stiffness, e.g. using recursive least squares (RLS) parameter estimation. The updated front and rear cornering stiffness are sent back to the first state observer processor, and are used by a second state observer processor for generating the estimated vehicle side-slip.
However, although U.S. Pat. No. 7,774,103 discloses a general concept for providing updated front and rear cornering stiffness's, the process of determining the updated front cornering stiffness and rear cornering stiffness by the state observer processor and the parameter estimation processor according to U.S. Pat. No. 7,774,103 is a slow loop, and provides the updated signals on the order of about one to two seconds.
Thus, although U.S. Pat. No. 7,774,103 suggest a slow loop process of determining updated front cornering stiffness and rear cornering stiffness estimates providing an improved ability to account for variations in tire cornering stiffness there is room for still further improvements.